May 12: MIT Medical answers your COVID-19 questions. Got a question about COVID-19? Send it to us at CovidQ@mit.edu, and we’ll do our best to provide an answer.
I’ve been hearing about a new “antigen test” for COVID-19. What is it, and how does it work?
The Food and Drug Administration (FDA) recently authorized the first antigen test to diagnose infection with SARS-CoV-2, the virus that causes COVID-19, which is why you’ve been hearing about it. The new test is from a company called Quidel Corp., but the FDA expects to authorize additional tests of this type in the near future.
If you or your child has ever had a rapid strep test, you’ve had firsthand experience with an antigen test. While other diagnostic tests for COVID-19, known as PCR tests, look for genetic material from the virus, the antigen test looks for molecules on the surface of the virus. PCR tests require expensive and specialized equipment and can take hours or days to get results. In contrast, antigen tests can be run in a lab or doctor’s office in about 15 minutes.
Unfortunately, there’s a trade-off between speed and accuracy, and antigen tests often fail to identify people who are actually infected, a shortcoming the FDA acknowledges. “Positive results from antigen tests are highly accurate,” the FDA writes, “but there is a higher chance of false negatives, so negative results do not rule out infection.” Just as your doctor may order a throat culture to definitively rule out strep throat when a rapid strep test comes back negative, the FDA recommends using the more sensitive COVID-19 molecular test for symptomatic individuals who test negative with antigen tests “prior to making treatment decisions or to prevent the possible spread of the virus due to a false negative.”
Some public officials anticipate using the COVID-19 antigen test as a tool for widespread screening, including the screening of asymptomatic people. Given the test’s speed and relatively low cost, the thought is that people could rapidly and easily be tested as a prerequisite to school attendance, work, or travel. But many medical experts, including Dr. Shawn Ferullo, MIT Medical’s chief of student health, see the test’s lack of sensitivity as precluding that type of use. However, he says, MIT Medical will be evaluating this test along with others, as they come on the market. “I think it is safe to say that we would use antigen testing if it is available and shows good data,” he says. “I envision it being a useful screening tool for patients who present with symptoms, the same way we use rapid strep tests or rapid flu tests. We know that all of these rapid tests miss people who are truly sick, but a positive result is almost always correct, and a more sensitive test is available as a follow-up for symptomatic patients who test negative.”